Fix logic for listing modules

[cmbuckley]

Logic for listing all modules includes the test directories within modules, which fails the workflow and creates unnecessary entries in dependabot.yml.

Decided to use -maxdepth 2, which does enforce the folder structure somewhat, but that's already the case because the changed_modules list looks at modules/*/*/*.tf. So we can solve both of those together if we ever need to.

Alternatives I considered that don't work:

• find modules -name main.tf -exec dirname {} \; - looks good but not every module has a main.tf
• find modules -name README.md -exec dirname {} \; - the test folders have readme files too

[robg-test]

I somewhat wonder if we'll ever need a Maxdepth more than 2.

E.g. Variations on EC2 / DB etc.

Happy for now